With a wedding on the way, Jessica Simpson is more determined than ever to stay thin! After giving birth to her son, Ace, last June, the 33-year-old returned as a spokesperson for Weight Watchers, and has since been showing off her slim figure.

In a new ad for the weight loss program, the mom of two flaunts her body in a tight, white dress, and shares how support from fellow Weight Watchers participants has helped her with the process. “When I started Weight Watchers, I wasn’t sure what to expect at the meetings,” she admits. “But I really love going. I do.”

“It reminds me — we don’t have to do this alone! It’s so much better to have some back-up and do it together, because we all face similar challenges,” she continues. “The meetings keep me focused and motivated. I have a newfound determination that I’m really proud of. I can honestly say, I’ve never been happier.”

Having two kids in two years delayed Jessica’s wedding to fiance Eric Johnson, but she’s recently said that they plan to marry before 2014 is up. Clock’s ticking, guys!